Obviously the real meat of this card is the damage protection during your turn. I would like to compare this to Sanctuary which also has powerful but conditional functions. Sanctuary is given HP regeneration to make it universal. I think it would be nice to make a parallel to Sanc and make Nature's Protection able to heal your creatures over time (like 1 HP per turn).
This way you don't have to worry about Sparks and Phase Salvagers while being useful for non-life decks. If you simply increase the HP, your opponent only needs one more mass CC to kill the creatures. But with HP regeneration, your opponent will be forced to gather mass CC and play all of them at once, thus delaying the mass CC and protecting the lower HP creatures (if your opponent wants to kill the high HP ones together with the weak ones)
In the past, I would have said that the spark and salvager issue was a pretty big deal... But, with the current SoPa buff the cat is out of the bag now (so to speak). So having the combo be viable via a non-rare card in a second element doesn't seem as big of a deal anymore.
This card's spark combo is actually a little more subdued really, since it doesn't provide any additional damage buff to them.
So now players have a choice, leave the sparks vulnerable but get instant damage, or allow them to grow at the cost of delayed results.
Right now, SoPa and Nightfall are the only two broad spectra stat buffs available, and nightfall is fairly situational. Having third option that is non-rare, but less situational than nightfall and more subdued than SoPa shouldn't be to bad.
That said, there is a good point in favor of a healing like effect. Namely, that the card could be made stackable without as much danger of becoming too powerful... of course, then you risk running guardian angel out of a job...
